Ionized Physical Vapor Deposition (iPVD) has been proposed for depositing films onto semiconductor wafer substrates, particularly where the sidewalls or bottoms of high aspect ratio sub-micron features are to be coated. The iPVD process is particularly proposed for the depositions of metals such as tantalum and copper. For iPVD in such applications, high-density low-potential plasma is produced in a sputter processing chamber to energize sputtered material, typically metal, that has been sputtered from a target by ions formed by a separate higher-potential plasma and accelerated across a plasma sheath at the target. The high-density low-potential plasma is employed to produce a high ion fraction or high percentage of ionized sputtered material, so that the coating material ions can be electrically attracted onto a substrate at nearly right angles to the substrate surface.
DC energy has been the primary energy source for the higher-potential plasma used for sputtering material from the target. Inductively coupled plasma (ICP) has been found to be useful in iPVD for energizing the high-density low-potential plasma used for ionizing the material that has been sputtered. In particular, iPVD devices and processes have been proposed having an ionized sputtered material source formed of an annular sputtering target with an RF energy source situated in the center of the target. Such sources are disclosed in U.S. Pat. Nos. 6,080,287 and 6,287,435, both hereby expressly incorporated by reference herein.
The sources of these iPVD devices and methods have provided features that are superior in many respects to other sources of the prior art. Such sources have been built with 5.5 kW RF power supplies that operate at a frequency of approximately 13.56 MHz, are relatively compact and not too expensive. Higher power than some such sources provide may be desired to couple more energy into the plasma and thereby increase the ion fraction in the plasma. Providing such higher power with a larger RF power supply of, for example, 10 kW, would currently add tens of thousands of dollars to the cost of the module, with additional costs also associated with the matching network and the increased cooling that would be required. Further, while 5 kW RF supplies are now built with solid state components, larger RF supplies often employ vacuum tubes, which are large and very heavy.
Furthermore, iPVD sources built according to the patents referenced above operate most effectively at higher pressures (above 60 mTorr). Depending on the application, use of higher pressures requires care to avoid gas phase nucleation, which can cause particles to form in the plasma, and makes the tolerances of the dark spaces more critical. At some point, increasing pressure can potentially cause a target to arc to an adjacent structure, which can result in significant damage and generate enormous quantities of particles that would contaminate the wafers to be processed.
Accordingly, it would be beneficial to efficiently increase the energy coupled to the plasma in an iPVD apparatus of the above patents. It would be further beneficial to equip such apparatus with the capability of operating at lower pressures while retaining the other advantages of such an apparatus.
A large number of sputtering sources have been devised for semiconductor wafer processing, and each has its own features. One such source that has features of the invention described below is the so-called Hollow Cathode Magnetron (HCM) described in U.S. Pat. No. 5,482,611. The apparatus in this patent is a sputter magnetron ion source for producing a high density plasma, which it generates in a cylindrical cathode cavity. Ions of target material are extracted from the cavity into a beam by producing a magnetic field cusp configuration with the magnetron magnets at a null region adjacent to the open end of the cathode cavity. The HCM source has been proposed and is currently used for iPVD.
One of the major drawbacks to the HCM source is that targets of the configuration proposed are extremely expensive. This is especially the case for metals such as tantalum. The large bucket type cathode of the HCM is difficult to fabricate and uses a large amount of expensive target grade material. Furthermore, in use, the material at the closed end of the cylindrical cavity in the target is eroded very little. In some HCM targets, there has been a net deposition at the closed end of the target, which, in addition to poor target utilization, has led to particle problems. To avoid such problems, a small rotating permanent magnet has been added to some HCM devices behind the target at the closed end of the cavity, which has added equipment cost and complexity. The HCM uses very high DC powers to obtain high ion fractions as the diameter of the cathode increases. This adds to the overall cost of the system and the cost of ownership. Further, using the DC power to the magnetron source to achieve plasma density means that the deposition rate and the ionization fraction are inextricably linked, where higher DC power gives a higher deposition rate and a higher ion fraction. A greater problem with the HCM than delivering the power to the cathode has been removing heat. The inventors of the HCM point out in their patent that the power required to operate the HCM increases as the cube of the diameter, while the cooling increases with the diameter squared. It would therefore be difficult to scale an HCM source to larger diameters while maintaining the same ion fractions as obtained in the smaller versions, due to the need to provide huge amounts of DC power and the difficulty in cooling the target sufficiently.
Accordingly, while the HCM has been useful in providing high energy plasma for iPVD systems, whether features of the HCM could be used to enhance the performance of an ICP source and, if so, how to combine the HCM and ICP features, have not been contemplated in the prior art.